Islanding detection in power converters

ABSTRACT

Methods and systems for islanding detection in power generators, particularly power converters. The present innovative methods can disconnect the power converter from the power distribution grid. Islanding detection can be performed by detecting the difference in resistive impedance between the islanded and un-islanded conditions, using an input-output power and input-output voltage correlation technique. The present methods can shut down the power converter instead of allowing it to continue running without connection to the grid, which will protect line-worker safety.

CROSS-REFERENCE

Priority is claimed from 61/765,131 filed Feb. 15, 2013, which is herebyincorporated by reference.

BACKGROUND

The present application relates to power generators and convertersconnected to power distribution grids and, more particularly, todetection of the disconnection between a power converter and the powerdistribution grid.

Note that the points discussed below may reflect the hindsight gainedfrom the disclosed inventions, and are not necessarily admitted to beprior art.

A new kind of power converter was disclosed in U.S. Pat. No. 7,599,196entitled “Universal power conversion methods,” which is incorporated byreference into the present application in its entirety. This patentdescribes a bidirectional (or multidirectional) power converter whichpumps power into and out of a link inductor which is shunted by acapacitor.

The switch arrays at the ports are operated to achieve zero-voltageswitching by totally isolating the link inductor+capacitor combinationat times when its voltage is desired to be changed. (When theinductor+capacitor combination is isolated at such times, the inductor'scurrent will change the voltage of the capacitor, as in a resonantcircuit. This can even change the sign of the voltage, without loss ofenergy.) This architecture has subsequently been referred to as a“current-modulating” or “Power Packet Switching” architecture.Bidirectional power switches are used to provide a full bipolar(reversible) connection from each of multiple lines, at each port, tothe rails connected to the link inductor and its capacitor. The basicoperation of this architecture is shown, in the context of thethree-phase to three-phase example of patent FIG. 1, in the sequence ofdrawings from patent FIG. 12a to patent FIG. 12j.

The ports of this converter can be AC or DC, and will normally bebidirectional (at least for AC ports). Individual lines of each port areeach connected to a “phase leg,” i.e. a pair of switches which permitthat line to be connected to either of two “rails” (i.e. the twoconductors which are connected to the two ends of the link inductor). Itis important to note that these switches are bidirectional, so thatthere are four current flows possible in each phase leg: the line cansource current to either rail, or can sink current from either rail.

Many different improvements and variations are shown in the basicpatent. For example, variable-frequency drive is shown (for controllinga three-phase motor from a three-phase power line), DC and single-phaseports are shown (patent FIG. 21), as well as three- and four-portsystems, applications to photovoltaic systems (patent FIG. 23),applications to Hybrid Electric vehicles (patent FIG. 24), applicationsto power conditioning (patent FIG. 29), half-bridge configurations(patent FIGS. 25 and 26), systems where a transformer is included (tosegment the rails, and allow different operating voltages at differentports) (patent FIG. 22), and power combining (patent FIG. 28).

Improvements and modifications of this basic architecture have also beendisclosed in U.S. Pat. Nos. 8,391,033, 8,295,069, 8,531,858, and8,461,718, all of which are hereby incorporated by reference.

The term “converter” has sometimes been used to refer specifically toDC-to-DC converters, as distinct from DC-AC “inverters” and/or AC-ACfrequency-changing “cycloconverters.” However, in the presentapplication the word converter is used more generally, to refer to allof these types and more, and especially to converters using acurrent-modulating or power-packet-switching architecture.

Islanding detection refers to detection of the disconnection of a powergenerator or converter from power distribution grids, when the powerconverter is still powered. Generally, power converters can still berunning even if the external connection to the distribution grid canhave been severed. This type of islanding can cause injuries to anelectric utility repairman since the circuit can still be powered.Islanding events must be quickly and reliably detected. Standards forsuch detection, notably IEEE-1S47.2003, which requires islanding eventdetection to be achieved, have been promulgated in the interest ofsafety of maintenance personnel and to avoid damage to the network, gridor various loads that can be connected to portions of the grid ornetwork. Therefore, there is a need for islanding detection that canquickly and reliably shut down the power converter when externalconnection to power distribution grid is lost.

SUMMARY

The present application teaches methods for islanding detection betweena power converter and a power distribution grid.

Islanding can be detected by the presence of higher or lower voltageconditions or frequency conditions in the distribution grid. The processstarts by determining when voltage drastically changes in the powerdistribution grid where an inverter can be connected. Thus, bysynchronizing a maximum power point tracking (MPPT) algorithm from theinverter to the calculation of DC power and AC voltages, it is possibleto determine if islanding from the power distribution grid has occurred.

The disclosed innovations, in various embodiments, provide one or moreof at least the following advantages. However, not all of theseadvantages result from every one of the innovations disclosed, and thislist of advantages does not limit the various claimed inventions.

-   -   Avoids circuit damage when sudden voltage changes occur in the        power distribution grid.    -   Improves protection for line worker safety.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments and whichare incorporated in the specification hereof by reference, wherein:

FIG. 1 illustrates an exemplary inverter with a three-phase AC outputconnected to a power distribution grid, according to an embodiment.

FIG. 2 depicts a flowchart of one sample method of islanding isolation,according to an embodiment.

FIG. 3A shows a simplified schematic of a sample power converter.

FIG. 3B shows sample voltage and current waveforms for a power cycle ofa sample power converter.

FIG. 3C shows an exemplary finite state machine for one sample controlarchitecture.

FIGS. 3D, 3E, and 3F show sample embodiments of output and inputvoltages.

FIG. 3G shows one sample embodiment of a bidirectional switch.

FIG. 3H shows one sample embodiment of a bidirectionalcurrent-modulating power converter.

FIGS. 3I, 3J, 3K, 3L, 3M, 3N, 3O, 3P, 3Q, and 3R show sample voltage andcurrent waveforms on an inductor during a typical cycle whiletransferring power at full load from input to output.

FIG. 3S shows voltage and current waveforms corresponding to the fullpower condition of FIGS. 3I-3R, with the conduction mode numberscorresponding to the mode numbers of FIGS. 3I-3R.

FIG. 3T shows an embodiment of the present inventions with a full bridgethree phase cycle topology, with controls and I/O filtering, including athree phase input line reactor as needed to isolate the small but highfrequency voltage ripple on the input filter capacitors from theutility.

FIG. 3U shows an embodiment of the present inventions with DC or SinglePhase portals.

FIG. 3V shows an embodiment of the present inventions with atransformer/inductor.

FIG. 3W shows an embodiment of the present inventions in a four portalapplication mixing single phase AC and multiple DC portals, as may beused to advantage in a solar power application.

FIG. 3X shows an embodiment of the present inventions in a three portalapplication mixing three phase AC portals and a DC portal, as may beused to advantage in a Hybrid Electric Vehicle application.

FIG. 3Y shows an embodiment of the present inventions as a Half-BridgeBuck-Boost Converter in a Single Phase AC or DC Topology with BCBS.

FIG. 3Z show a sample embodiment in a Half-Bridge Buck-Boost Converterin a Three Phase AC Topology with BCBS.

FIG. 3AA shows a sample embodiment in a single phase to three phasesynchronous motor drive.

FIG. 3BB shows a sample embodiment with dual, parallel, “power modules”,each of which consists of 12 bi-directional switches and a parallelinductor/capacitor. More than two power modules may of course be usedfor additional options in multiway conversion.

FIG. 3CC shows an embodiment of the present inventions as a three phasePower Line Conditioner, in which role it can act as an Active Filterand/or supply or absorb reactive power to control the power factor onthe utility lines.

FIG. 3DD shows a sample schematic of a microgrid embodiment.

FIG. 3EE shows another sample embodiment of a microgrid.

DETAILED DESCRIPTION OF SAMPLE EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to presently preferred embodiments(by way of example, and not of limitation). The present applicationdescribes several inventions, and none of the statements below should betaken as limiting the claims generally.

Some exemplary parameters will be given to illustrate the relationsbetween these and other parameters. However it will be understood by aperson of ordinary skill in the art that these values are merelyillustrative, and will be modified by scaling of further devicegenerations, and will be further modified to adapt to differentmaterials or architectures if used.

Referring initially to FIG. 3H, illustrated is a schematic of a samplethree phase converter 100 that embodies the present inventions. Theconverter 100 is connected to a first and second power portals 122 and123 each of which can source or sink power, and each with a port foreach phase of the portal. Converter 100 can transfer electric powerbetween said portals while accommodating a wide range of voltages,current levels, power factors, and frequencies between the portals.

Said first portal can be for example, a 460 VAC three phase utilityconnection, while said second portal can be a three phase inductionmotor which is to be operated at variable frequency and voltage so as toachieve variable speed operation of said motor. The present inventionscan also accommodate additional portals on the same inductor, as can bedesired to accommodate power transfer to and from other power sourcesand/or sinks, as shown in FIGS. 3W and 3X.

Referring to FIG. 3H, converter 100 is comprised of a first set ofelectronic switches S_(1u), S_(2u), S_(3u), S_(4u), S_(5u), and S_(6u)that are connected between a first port 113 of a link inductor 120 andeach phase, 124 through 129, of the input portal, and a second set ofelectronic switches S_(1l) S_(2l), S_(3l), S_(4l), S_(5l), and S_(6l)that are similarly connected between a second port 114 of link inductor120 and each phase of the output portal. A link capacitor 121 isconnected in parallel with the link inductor, forming the linkreactance. Each of these switches is capable of conducting current andblocking current in both directions, as seen in e.g. FIG. 3G. Many othersuch bi-directional switch combinations are also possible.

The converter 100 also has input and output capacitor filters 130 and131, respectively, which smooth the current pulses produced by switchingcurrent into and out of inductor 120. Optionally, a line reactor 132 canbe added to the input to isolate the voltage ripple on input capacitorfilter 131 from the utility and other equipment that can be attached tothe utility lines. Similarly, another line reactor, not shown, can beused on the output if required by the application.

For illustration purposes, assume that power is to be transferred in afull cycle of the inductor/capacitor from the first to the secondportal, as is illustrated in FIG. 3S. Also assume that, at the instantthe power cycle begins, phases A_(i) and B_(i) have the highest line toline voltage of the first (input) portal, link inductor 120 has nocurrent, and link capacitor 121 is charged to the same voltage as existsbetween phase A_(i) and B_(i). The controller FPGA 1500, shown in FIG.3T, now turns on switches S_(1u) and S_(2l), whereupon current begins toflow from phases A_(i) and B_(i) into link inductor 120, shown as Mode 1of FIG. 3I.

FIG. 3S shows the inductor current and voltage during the power cycle ofFIGS. 3I-3R, with the Conduction Mode sequence 1300 corresponding to theConduction Modes of FIGS. 3I-3R. The voltage on the link reactanceremains almost constant during each mode interval, varying only by thesmall amount the phase voltage changes during that interval. After anappropriate current level has been reached, as determined by controller1500 to achieve the desired level of power transfer and currentdistribution among the input phases, switch S_(2l) is turned off.

Current now circulates, as shown in FIG. 3J, between link inductor 120and link capacitor 121, which is included in the circuit to slow therate of voltage change, which in turn greatly reduces the energydissipated in each switch as it turns off. In very high frequencyembodiments of the present inventions, the capacitor 121 can consistsolely of the parasitic capacitance of the inductor and/or other circuitelements. (Note that a similar process is shown in FIG. 3O.)

To continue with the cycle, as shown as Mode 2 in FIG. 3K and FIG. 3S,switch S_(3l) is next enabled, along with the previously enabled switchS_(1u). As soon as the link reactance voltage drops to just less thanthe voltage across phases A_(i) and C₁, which are assumed for thisexample to be at a lower line-to-line voltage than phases A_(i) andB_(i), switches S_(1u) and S_(3l) become forward biased and start tofurther increase the current flow into the link inductor, and thecurrent into link capacitor temporarily stops.

The two “on” switches, S_(1u) and S_(3l), are turned off when thedesired peak link inductor current is reached, said peak link inductorcurrent determining the maximum energy per cycle that can be transferredto the output. The link inductor and link capacitor then again exchangecurrent, as shown if FIG. 3J, with the result that the voltage on thelink reactance changes sign, as shown in graph 1301, between modes 2 and3 of FIG. 3S. Now as shown in FIG. 3L, output switches S_(5u) and S_(6l)are enabled, and start conducting inductor current into the motor phasesA_(o) and B_(o), which are assumed in this example to have the lowestline-to-line voltages at the present instance on the motor.

After a portion of the inductor's energy has been transferred to theload, as determined by the controller, switch S_(5u) is turned off, andS_(4u) is enabled, causing current to flow again into the linkcapacitor. This increases the link inductor voltage until it becomesslightly greater than the line-to-line voltage of phases A_(o) andC_(o), which are assumed in this example to have the highestline-to-line voltages on the motor. As shown in FIG. 3M, most of theremaining link inductor energy is then transferred to this phase pair(into the motor), bringing the link inductor current down to a lowlevel.

Switches S_(4u) and S_(6l) are then turned off, causing the linkinductor current again to be shunted into the link capacitor, raisingthe link reactance voltage to the slightly higher input line-to-linevoltage on phases A_(i) and B_(i). Any excess link inductor energy isreturned to the input. The link inductor current then reverses, and theabove described link reactance current/voltage half-cycle repeats, butwith switches that are compeimentary to the first half-cycle, as isshown in FIGS. 3N-3R, and in Conduction Mode sequence 1300, and graphs1301 and 1302. FIG. 3O shows the link reactance current exchange duringthe inductor's negative current half-cycle, between conduction modes.

Note that TWO power cycles occur during each link reactance cycle: withreference to FIGS. 3I-3R, power is pumped IN during modes 1 and 2,extracted OUT during modes 3 and 4, IN again during modes 5 and 6(corresponding to e.g. FIG. 3P), and OUT again during modes 7 (as ine.g. FIG. 3Q) and 8. The use of multi-leg drive produces eight modesrather than four, but even if polyphase input and/or output is not used,the presence of TWO successive in and out cycles during one cycle of theinductor current is notable.

As shown in FIGS. 3I-3S, Conduction Mode sequence 1300, and in graphs1301 and 1302, the link reactance continues to alternate between beingconnected to appropriate phase pairs and not connected at all, withcurrent and power transfer occurring while connected, and voltageramping between phases while disconnected (as occurs between the closelyspaced dashed vertical lines of which 1303 in FIG. 3S is one example.

In general, when the controller 1500 deems it necessary, each switch isenabled, as is known in the art, by raising the voltage of the gate 204on switch 200 above the corresponding terminal 205, as an example.Furthermore, each switch is enabled (in a preferred two gate version ofthe switch) while the portion of the switch that is being enabled iszero or reverse biased, such that the switch does not start conductionuntil the changing link reactance voltage causes the switch to becomeforward biased. Single gate AC switches can be used, as with a one-wayswitch embedded in a four diode bridge rectifier, but achievingzero-voltage turn on is difficult, and conduction losses are higher.

In FIG. 3T, current through the inductor is sensed by sensor 1510, andthe FPGA 1500 integrates current flows to determine the current flowingin each phase (port) of the input and output portals. Phase voltagesensing circuits 1511 and 1512 allow the FPGA 1500 to control whichswitches to enable next, and when.

FIGS. 3I-3R shows current being drawn and delivered to both pairs ofinput and output phases, resulting in 4 modes for each direction of linkinductor current during a power cycle, for a total of 8 conduction modessince there are two power cycles per link reactance cycle in thepreferred embodiment. This distinction is not dependent on the topology,as either three phase converter can be operated in either 2 modes or 4conduction modes per power cycle, but the preferred method of operationis with 4 conduction modes per power cycle, as that minimizes input andoutput harmonics.

For single phase AC or DC, it is preferred to have only two conductionmodes per power cycle, or four modes per link reactance cycle, as thereis only one input and output pair in that case. For mixed situations, asin the embodiment of FIG. 3X which converts between DC or single phaseAC and three phase AC, there can be 1 conduction mode for the DCinterface, and 2 for the three phase AC, for 3 conduction modes perpower cycle, or 6 modes per link reactance cycle. In any case, however,the two conduction modes per power half-cycle for three phase operationtogether give a similar power transfer effect as the singe conductionmode for single phase AC or DC.

Another sample embodiment of the present inventions is shown in FIG. 3U,which shows a single phase AC or DC to single phase AC or DC converter.Either or both input and output can be AC or DC, with no restrictions onthe relative voltages. If a portal is DC and can only have power floweither into or out of said portal, the switches applied to said portalcan be uni-directional. An example of this is shown with thephotovoltaic array of FIG. 3W, which can only source power.

FIG. 3V shows an embodiment of the present inventions as a FlybackConverter. The circuit of FIG. 3U has been modified, in that the linkinductor is replaced with a transformer 2200 that has a magnetizinginductance that functions as the link inductor. Any embodiment of thepresent inventions can use such a transformer, which can be useful toprovide full electrical isolation between portals, and/or to providevoltage and current translation between portals, as is advantageous, forexample, when a first portal is a low voltage DC battery bank, and asecond portal is 120 volts AC, or when the converter is used as anactive transformer.

In the embodiments of the present inventions shown in FIGS. 3W and 3X,the number of portals attached to the link reactance is more than two,simply by using more switches to connect in additional portals to theinductor. As applied in the solar power system of FIG. 3W, this allows asingle converter to direct power flow as needed between the portals,regardless of their polarity or magnitude.

Thus, in one sample embodiment, the solar photovoltaic array can be atfull power, e.g. 400 volts output, and delivering 50% of its power tothe battery bank at e.g. 320 volts, and 50% to the house AC at e.g. 230VAC. Prior art requires at least two converters to handle thissituation, such as a DC-DC converter to transfer power from the solar PVarray to the batteries, and a separate DC-AC converter (inverter) totransfer power from the battery bank to the house, with consequentialhigher cost and electrical losses. The switches shown attached to thephotovoltaic power source need be only one-way since the source is DCand power can only flow out of the source, not in and out as with thebattery.

In the sample power converter of FIG. 3X, as can be used for a hybridelectric vehicle, a first portal is the vehicle's battery bank, a secondportal is a variable voltage, variable speed generator run by thevehicle's engine, and a third portal is a motor for driving the wheelsof the vehicle. A fourth portal, not shown, can be external single phase230 VAC to charge the battery. Using this single converter, power can beexchanged in any direction among the various portals. For example, themotor/generator can be at full output power, with 50% of its power goingto the battery, and 50% going to the wheel motor. Then the driver candepress the accelerator, at which time all of the generator power can beinstantly applied to the wheel motor. Conversely, if the vehicle isbraking, the full wheel motor power can be injected into the batterybank, with all of these modes using a single converter.

FIGS. 3Y and 3Z show half-bridge converter embodiments of the presentinventions for single phase/DC and three phase AC applications,respectively. The half-bridge embodiment requires only 50% as manyswitches, but results in 50% of the power transfer capability, and givesa ripple current in the input and output filters which is about doublethat of the full bridge implementation for a given power level.

FIG. 3AA shows a sample embodiment as a single phase to three phasesynchronous motor drive, as can be used for driving a householdair-conditioner compressor at variable speed, with unity power factorand low harmonics input. Delivered power is pulsating at twice the inputpower frequency.

FIG. 3BB shows a sample embodiment with dual, parallel power modules,with each module constructed as per the converter of FIG. 3H, excludingthe I/O filtering. This arrangement can be advantageously used wheneverthe converter drive requirements exceed that obtainable from a singepower module and/or when redundancy is desired for reliability reasonsand/or to reduce I/O filter size, so as to reduce costs, losses, and toincrease available bandwidth.

The power modules are best operated in a manner similar to multi-phaseDC power supplies such that the link reactance frequencies are identicaland the current pulses drawn and supplied to the input/output filtersfrom each module are uniformly spaced in time. This provides for a moreuniform current draw and supply, which can greatly reduce the per unitfiltering requirement for the converter. For example, going from one totwo power modules, operated with a phase difference of 90 degreesreferenced to each of the modules inductor/capacitor, produces a similarRMS current in the I/O filter capacitors, while doubling the ripplefrequency on those capacitors. This allows the same I/O filtercapacitors to be used, but for twice the total power, so the per unitI/O filter capacitance is reduced by a factor of 2. More importantly,since the ripple voltage is reduced by a factor of 2, and the frequencydoubled, the input line reactance requirement is reduced by 4, allowingthe total line reactor mass to drop by 2, thereby reducing per unit linereactance requirement by a factor of 4.

FIG. 3CC shows a sample embodiment as a three phase Power LineConditioner, in which role it can act as an Active Filter and/or supplyor absorb reactive power to control the power factor on the utilitylines. If a battery, with series inductor to smooth current flow, isplaced in parallel with the output capacitor 2901, the converter canthen operate as an Uninterruptible Power Supply (UPS).

FIG. 3A shows an example of a circuit implementing this architecture. Inthis example, one port is used for connection to the AC grid (or otherthree-phase power connection). The other is connected to a motor, toprovide a variable-frequency drive.

In FIG. 3A, an LC link reactance is connected to two DC portals havingtwo ports (e.g. lines) each, and to a three-phase AC portal. Each portconnects to a pair of bidirectional switches, such that onebidirectional switch connects the respective port to a rail at one sideof the link reactance and the other bidirectional switch connects theport to a rail at the other side of the link reactance.

In one sample embodiment, voltage and current across a link reactancecan be seen in, e.g., FIG. 3B. Link voltage waveform 1301 and linkcurrent waveform 1302 correspond to an arbitrary set of inputs andoutputs. After a conduction interval begins and the relevant switchesare activated, voltage 1301 on the link reactance remains almostconstant during each mode interval, e.g. during each of modes 1-8. Afteran appropriate current level has been reached for the present conductionmode, as determined by the controller, the appropriate switches areturned off. This can correspond to, e.g., conduction gap 1303. Theappropriate current level can be, e.g., one that can achieve the desiredlevel of power transfer and current distribution among the input phases.

Current can now circulate between the link inductor and the linkcapacitor, which is included in the circuit to slow the rate of voltagechange. This in turn greatly reduces the energy dissipated in eachswitch as it turns off After the link voltage reaches appropriate levelsfor the next set of ports, the appropriate switches are enabled, andenergy transfer between the portal and the link continues with the nextline pair.

A power converter according to some embodiments of this architecture canbe controlled by, e.g., a Modbus serial interface, which can read andwrite to a set of registers in a field programmable gate array (FPGA).These registers can define, e.g., whether a portal is presently aninput, an output, or disabled. Power levels and operation modes can alsobe determined by these registers.

In some embodiments, a DC portal preferably has one line pair, whereeach line pair is e.g. a pair of ports that can transfer energy to orfrom the link reactance through semiconductor switches. A three-phase ACportal will always have three lines, and will often have a fourth(neutral), but only two will be used for any given power cycle.

Given three lines, there are three possible two-line combinations. Forexample, given lines A, B, and C, the line pairs will be A-B, B-C, andA-C. At least one of these line pairs will be configured as an input andat least one line pair will be configured as an output.

Register values for each portal can be used to determine the amount ofcharge, and then the amount of energy, to be transferred to or from eachportal during each conduction period. An interface then controls eachportal's switches appropriately to transfer the required charge betweenthe link and the enabled portals.

A separate set of working registers can be used in some embodiments tocontrol converter operations. Any value requiring a ramped rate ofchange can apply the rate of change to the working registers.

The mode set for a portal during a given power cycle can determine whatfactor will drive the portal's power level. This can be, for example,power, current, conductance, or net power. In “net power” mode, theportal's power level can be set by, e.g., the sum of other portal'spower settings. The mode of at least one portal will most preferably beset to net power in order to source or sink the power set by the otherportals. If two portals are set as net power, the two portals will sharethe available power.

A main control state machine and its associated processes can controlthe transfer of power and charge between portals, as seen in FIG. 3C.The state machine can be controlled in turn by the contents ofregisters. The state machine transfers the amount of energy set by theinterface from designated input portals to the link reactance, and thentransfers the appropriate amount of energy from the link to designatedoutput portals.

The Reset/Initialize state occurs upon a power reset, when converterfirmware will perform self-tests to verify that the converter isfunctioning correctly and then prepare to start the converter. If nofaults are found, the state machine proceeds to the Wait_Restart state.

The Wait_Restart state can be used to delay the start of the converterupon power up or the restart of the converter when certain faults occur.If a fault occurs, a bleed resistor is preferably engaged. Certainfaults, once cleared, will preferably have a delay before restartingnormal converter operation. The next state will be Startup.

When the Startup state begins, there is no energy in the link. Thisstate will put enough energy into the link to resonate the link to theoperational voltage levels, which are preferably greater than thehighest voltage of any input line pair.

When starting from an AC portal, the firmware will wait until a zerovoltage crossing occurs on a line pair of the AC portal. The firmwarewill then wait until the voltage increases to about 40 volts, then turnon the switches of the line pair for a short duration. This will putenergy into the link and start the link resonating. The peak resonantvoltage must be greater than the AC line pair for the next cycle. Afterthe first energy transfer, more small energy transfers can be made tothe link as the link voltage passes through the line pair voltage,increasing the link's resonant voltage until the link's peak voltage isequal to or greater than the first input line pair voltage. At thispoint, a normal power cycle is ready to start and the state will changeto Power Cycle Start upon detection of a zero current crossing in thelink.

In the Power Cycle Start state, the amount of charge and energy thatwill be transferred to or from the link and each portal is determined atthe start of a power cycle. This state begins on a link zero currentcrossing detection, so the link current will be zero at the start of thestate. The link voltage will preferably be equal or greater than thehighest input voltage.

The input and output line pairs that are not disabled will be sorted bytheir differential voltages from the highest voltage to the lowestvoltage, where outputs are defined as having a negative voltage withrespect to the start of the current power cycle. If the power factor ofthe AC portal is not unity, one of the two line pairs of the AC portalwill switch between input and output for a portion of a 60 Hz waveform.

If a DC portal's mode is set to have constant current or constant power,the constant current or power levels are converted to equivalentconductance values and used to adjust the relevant portal's settingsappropriately. If the portal's mode is set to net power, the portal willtransfer the sum of all the energy of all other portals not in net powermode.

MPPT (Maximum Power Point Tracking) mode preferably constantly adjuststhe charge put into the Link from a photovoltaic array to maximizetransferred energy. There will typically be a maximum current draw afterwhich voltage begins to decrease, where the particular maximal currentdepends on the photovoltaic array's output characteristics. This maximalcurrent corresponds to maximum power, beyond which point energy transferwill decline. To determine this maximal point, energy transfer can bemonitored while conductance is adjusted until a local maximum is found.There can be some variations in the amount of energy delivered, but thiswill tend to maximize energy transfer.

The charge Q to be transferred to the link can be found as, e.g., theproduct of conductance G, voltage V, and link power cycle period T (i.e.Q=G*V*T). The energy E to be transferred is then simply the product ofthe voltage times the charge (E=V*Q=G*V²*T).

Since other portal operation modes prescribe the energy to betransferred to or from the link, at least one portal is most preferablyin net power mode. At least one portal is most preferably thus dependenton the energy in the link, rather than prescribing the same, so that theamount of energy put into the link equals the amount of energy taken outof the link.

The amount of energy that is put into the link by other modes is summedtogether to determine the energy transfer to or from portals operatingin net power mode. A small amount of energy can in some cases besubtracted from this sum if extra energy is to be added to the link thiscycle. If multiple portals are operating in net power mode, theavailable energy is preferably split between the two ports according to,e.g., the Modbus registers. The amount of charge to be transferred ispreferably determined by the relationship charge=energy/voltage.

For an AC portal, the phase angle between the voltage and current on theAC portal can be varied, based on e.g. power factor settings. An ACportal can also source reactive current for AC portal filter capacitorsto prevent the filter capacitors from causing a phase shift.

Three-phase charge calculations for a three-phase AC portal can, in someembodiments, proceed as follows. Zero crossing of the AC voltagewaveform for a first phase is detected when the voltage changes from anegative to positive. This can be defined as zero degrees, and a phaseangle timer is reset by this zero crossing. The phase angle timer ispreferably scaled by the measured period of the AC voltage to derive theinstantaneous phase angle between the voltage of this first phase andthe zero crossing. The instantaneous phase angle can then be used toread the appropriate sinusoidal scalar from a sinusoidal table for thefirst phase. The instantaneous phase angle can then be adjustedappropriately to determine the sinusoidal scalars for the second andthird phases.

The instantaneous phase angle of the first phase can be decremented bye.g. 90° to read a reactive sinusoidal scalar for the first phase, andthen adjusted again to determine reactive sinusoidal scalars for theother two phases.

The required RMS line current of the portal can then be determined, butcan differ dependent on, e.g., whether the portal is in net power modeis controlled by conductance. In conductance mode, RMS line current canbe found by, e.g., multiplying the conductance for the AC portal by itsRMS voltage.

In net power mode, RMS line current can be found e.g. as follows. Theenergy transferred to the link by all portals not in net power mode isfirst summed to determine the net power energy available. The smallamount of energy defined by the link energy management algorithm can besubtracted from the available energy if relevant. The net energyavailable is multiplied by the percentage of total power to be allocatedto the present portal, which is 100% if only one portal is in net powermode: Power=Σ Energy*portal %.

Line RMS current can then be found by dividing the energy for the ACport by the RMS voltage of the portal, the link power cycle period, andsquare root of 3: linecurrent_(rms)=Power/(time_(link cycle)*voltage_(rms)*√3).

The instantaneous in-phase current can then be calculated, and willagain differ based on the operational mode of the portal. In aconductance mode, the three line-to-line instantaneous voltages can bemultiplied by the portal conductance to determine the instantaneouscurrent of each phase.

In net power mode, the sinusoidal scalar for each phase can bemultiplied by the RMS line current to determine the instantaneouscurrent of each phase. Alternately, voltages from an analog/digitalconverter can be used to find the instantaneous currents directly:Instantaneous Current=energy*V_(a/d)/(3*period*Vrms²). The charge canthen be found as Q=energy*V_(a/d)/(3*Vr_(ms) ²).

RMS line reactive current can then be found e.g. from power factor asfollows: Power Factor=Power/(Power+reactive power)

reactive power=(Power/power factor)−Power

reactive power_(line to line)=Power/(3*power factor)−Power/3

rms reactive current_(line)=reactive power_(line to line)/rmsvoltage_(line to line).

Filter capacitive current can then be calculated from the filtercapacitance values, line to line voltage, and frequency. Capacitivecompensation current can then be added to the RMS line reactive currentto determine the total RMS line reactive current. Total RMS reactivecurrent can then be multiplied by the reactive sinusoidal scalar toderive the instantaneous reactive current for each phase.

The instantaneous current and the instantaneous current for each phasecan then be added together and multiplied by the period of the linkpower cycle to determine the amount of charge to be transferred for eachphase.

The energy to transfer to or from the link can be found by multiplyingthe charge value of each phase by the instantaneous voltage and summingthe energy of the three phases together.

The phase with the largest charge will be dominant phase for this cycle,and the two line pairs for the AC port will be between the dominantphase and each of the other two phases. The amount of charge to betransferred for each line pair will be the amount of charge calculatedfor the non-dominant line of the pair. The next state will be the ChargeTransfer state.

In the Charge Transfer state, a first line pair is selected and therespective switches turned on. Even though the switches are on, noconduction will occur until the voltage of the link drops below that ofan input line pair, or rises above the voltage of an output line pairwhere appropriate. If one end of the link inductor reaches the voltageof one line of the line pair, that end of the link inductor isindirectly anchored to the respective line. The link inductor willsubsequently not change in voltage until the respective switch is turnedoff.

The voltage of the line pair is then compared to the integrated linkvoltage. It is generally assumed that current will begin to flow throughthe switches once the integrated link voltage reaches the voltage of theline pair, minus a switch voltage drop. This switch voltage drop isassumed to be on the order of e.g. 8 V for a pair of switches.

The amount of charge flowing into or out of the link is monitored. Thecharge can be found as Q=ΣIΔt, or the sum of the current times the timeinterval.

The link current is typically approximately zero at the start of a powercycle. The link current increases through the end of the last input,then decreases until reaching zero at the beginning of the next powercycle. The link current can be found as I=Σ(V_(instantaneous) Δt/L), orthe sum of the instantaneous voltage times the time interval divided bythe inductance.

When the transferred charge is determined to have met the calculatedamount for the given line pair, the state machine can progress to thenext state. The next state can be Common Mode Management, or can beIdle. If the next state is Idle, all switches are turned off. In somesample embodiments, the state machine will only progress to the CommonMode Management state after the final output line pair.

The Common Mode Management state controls the common mode voltage of thelink, as well as the energy left in the link following the prior state.To control the common mode voltage, one of the switches for the priorline pair is turned off, while the other switch is controlled by theCommon Mode Management state. By having one switch on, the adjacent endof the link can be anchored at the respective line voltage. The voltageat the opposite end of the link can then increase until the currentthrough the inductor drops to zero. The remaining switch can then beturned off. When a zero crossing is detected in the link current, thestate machine will progress to the Idle state.

Two types of anchoring can be used in Common Mode Management. Directanchoring occurs when one switch of a line pair is closed (turned on),which fixes the voltage of the nearest end of the link to the respectiveline voltage. While this switch is turned on, any change to the link'sdifferential voltage will occur on the other end of the link, which willin turn change the link's common mode voltage.

Indirect anchoring occurs when both of a line pair's switches are turnedon prior to a charge transfer. When the voltage of one end of the linkis one switch-voltage-drop below the corresponding line voltage, therespective end of the link is anchored to that voltage. The voltage ofthe other end of the link will continue to change until the voltageacross the link is equal to two switch-voltage-drops below the line pairvoltage. At this point, charge transfer between the link and the linepair begins.

The Common Mode Management state also controls the energy left in thelink after output charge transfer is completed, or after ramp-up. Afterthe last output charge transfer, enough energy will most preferablyremain in the link to have completed the last output charge transfer,and to cause the link voltages first to span, and then to decrease tojust below, the voltages of the first input line pair. This can permitzero-voltage switching of the input switches. Zero-voltage switching, inturn, can reduce switching losses and switch overstressing. The voltagesacross the switches when conduction begins can preferably be e.g. 4 V,but is most preferably no more than 20 V. If insufficient energy remainsin the link to permit zero-voltage switching, a small amount of powercan be transferred from one or more ports in net power mode to the linkduring the subsequent power cycle.

FIG. 3D shows a sample embodiment in which the voltages of the lastoutput span the voltages of the first input. It can be seen that thelink-energy requirements have been met, though small amounts of energycan occasionally be needed to account for link losses.

FIG. 3E shows another sample embodiment in which the voltages of thelast output are spanned by the voltages of the first input. Enoughenergy must be maintained in the link to resonate the link voltages toabove the voltages of the first input. Additional energy can sometimesbe needed to account for small link losses, but the link-energyrequirements can be met fairly easily.

FIG. 3F shows a third sample embodiment, in which the voltages of thelast output neither span nor are spanned by the voltages of the firstinput. Since the last output voltages do not span the first inputvoltages, the link voltage will need to be increased. Enough energy inthe link needs to be maintained in the link to resonate the linkvoltages above the voltages of the first input pair before the linkcurrent crosses zero. This can in some sample embodiments require smallamounts of additional energy to fulfill this requirement.

In each of the sample embodiments of FIGS. 3D-3F, the common modevoltage of the link will preferably be forced toward the common modevoltage of the first input. The switch of the last output furthest involtage from the common mode voltage will preferably be turned offfirst. The link will thus first anchor to the end with a voltage closestto that desired while the other end changes. The other switch ispreferably turned off either once the common mode voltage of the firstinput is turned off or else a zero-crossing is detected in the linkcurrent.

The Idle State most preferably ensures that all link switches remain fora period of time immediately after a switch is turned off. As switchesdo not turn off instantaneously, this can be used to minimizecross-conduction between lines, which can occur when one switch isturned on before another has time to completely turn off. In some sampleembodiments in which the switches comprise e.g. IGBTs, the time betweennominal and actual turn-off of the switches can be significant. Afterthe requisite time has elapsed, the state machine can advance to thenext state. If the prior state was the last line pair, the next state ispreferably the Power Cycle Start state, and is otherwise preferably theCharge Transfer state.

In one sample embodiment, the bidirectional switches can comprise, e.g.,two series IGBTs and two parallel diodes, as in FIG. 3G. In anembodiment like that of FIG. 3G, a bidirectional switch can have twocontrol signals, each controlling one direction of current flow. Otherbidirectional switches are also possible.

Switch control signals are most preferably monitored to preventcombinations of switches being turned which can lead to catastrophicfailures of the converter. Only switches corresponding to a single linepair will preferably be enabled at a time. As relatively few possibleswitch combinations will prevent catastrophic failure, monitoring canlook for the few permissible combinations to allow instead of lookingfor the many combinations to forbid.

Switch control signals can preferably also be monitored to avoid turningnew switches on too quickly after another switch has been turned off.The switches take a finite time to turn off, and turning on anotherswitch too quickly can cause damaging cross-conduction.

Voltage across each switch is also preferably monitored before it isturned on to avoid damaging overvoltage.

Zero-crossings in the link current are preferably detected e.g. using atoroid installed on a link cable. Instead of directly measuring linkcurrent, it can be calculated by integrating the voltage across the linkand scaling the result. This calculated current can preferably be resetevery time a zero-crossing is detected, to prevent long-termaccumulation of error. Zero-crossings, when detected, can also be usedto set the link polarity flag, as the voltage across the link reverseswhen the direction of current flow changes.

In some sample embodiments, power converter voltages can be measuredwith high-speed serial analog-to-digital (A/D) converters. In one sampleembodiment, these converters can have e.g. a 3 MSPS (mega-samples persecond) conversion rate. In one sample embodiment, the converters cantake e.g. 14 clocks to start a conversion and clock in the serial data,leading to e.g. a data latency of 0.3 μs. One sample embodiment can usee.g. 22 such A/D converters.

Islanding occurs when a converter continues to output power when the ACpower grid goes down. This can be extremely dangerous, especially forline crews attempting to fix the AC power grid. Islanding conditions aremost preferably detected and used to trigger a shutdown of theconverter's AC output.

Ground faults will most preferably be detected on DC inputs. When DCcontactors are closed, the voltage drop between the common connection ofa portal's connectors and the DC portal's ground connection willpreferably be measured. If this voltage is over a certain limit, eithertoo much ground current is present or else the portal's ground fuse isblown. Both of these situations will generate a fault.

A fault will preferably be generated if toroids on input cables detectsurges.

Each DC portal will preferably have a pair of contactors connectingpositive and negative power sources to an input ground connection.Configuration information is preferably read from the registers and usedto open or close the contactors as needed. Before contactors are closed,DC filter capacitors are preferably pre-charged to the voltage on theline side of the contactors in order to prevent high-current surgesacross the contacts of the contactors.

An LCD or other type of screen is preferably provided as an interface toa power converter.

The temperature of a heat sink is preferably monitored and used todirect fans. Tachometers on the fans can preferably be monitored, andthe information used to shut down fan control lines if a fan fails. Asthese temperature sensors can occasionally give incorrect information,in some sample embodiments e.g. two preceding readings can be comparedagainst the current temperature reading, and e.g. the median value canbe chosen as the current valid temperature.

In some sample embodiments, a processor can be used to control a powerconverter. This can be e.g. a NIOS processor which is instantiated inthe field-programmable gate array.

In some sample embodiments, an interface to e.g. a 1 GB flash RAM can beused. In one sample embodiment, a flash RAM can have e.g. a 16-bit-widebus and e.g. a 25-bit address bus. In some sample embodiments, an activeserial memory interface can permit reading from, writing to, or erasingdata from a serial configuration flash memory.

In some sample embodiments, a field-programmable gate array can beconnected to e.g. a 1 MB serial nvSRAM with real time clock.

In some sample embodiments, dual row headers on a pc board can be usede.g. for testing and debugging purposes.

In some sample embodiments, LEDs or other indicators can be present on acontrol board. These indicators can be used e.g. for diagnosticpurposes.

To minimize risks of condensation or other types of moisture damagingelectronics, a power converter can preferably be kept in a sealedcompartment. Some air flow is often necessary, however, due to e.g.temperature changes over time. Any air flowing into or out of theconverter most preferably passes through one or more dehumidifiers. Ifleft alone, the dehumidifiers eventually saturate and become useless orworse. Instead, heating elements can preferably be included withdehumidifiers to drive out accumulated moisture. When air flows into theotherwise-sealed compartment, dehumidifiers can remove moisture. Whenair flows out of the compartment, the heating elements can activate, sothat ejected moisture is carried away with the outflowing air instead ofcontinuing into the converter.

As used herein, “point of isolation” can refer to an exact point betweenan inverter and a power distribution grid, where the inverter can betotally disconnected from the grid.

FIG. 1 illustrates an inverter with a three-phase AC 100. The circuitcan include DC+ 102 and DC− 104 rails to inverter 106. Inverter 106 canbe connected to a photovoltaic array, wind turbine or other kind ofpower source. Inverter 106 can be further connected to three-phase AC108.

Inverter 106 can also include voltage or current sensors or both (notshown), to sense voltages and current in the different phases ofinverter 106. Sensors can provide information about input and outputvoltage or current, or both. Thereafter, this data can be sent to an A/Dconverter 114, converting analog information into digital informationfor an field-programmable gate array (FPGA) controller 116 to analyze.Then, through calculations, waveform look-up tables and controlalgorithms embedded in the FPGA controller 116, the FPGA controller 116,can select suitable switches (not shown), to open and close connectionsfrom three-phase AC 108 to the DC+ 102 and DC− 104 rails. Additionally,a user interface 118 can be connected to the FPGA controller 116, toallow a user to monitor voltage or current waveforms, or both, and toinput data necessary for control of the inverter 106.

Three-phase AC 108 can be connected to the power distribution grid 110through a point of isolation 120, where three-phase AC 108 can beislanded from power distribution grid 110. Power distribution grid 110can be further connected to electric company 122, which can distributepower to a particular region.

FIG. 2 depicts a flowchart of an embodiment of a method 200 of detectingislanding isolation. Some conditions for the processes described hereinmay apply, such that first, a maximum power point tracking (MPPT)algorithm makes continual changes in DC power draw, and second,measurement of AC/DC voltages from sensor data is made, which allowsinference of AC and DC current and power.

A suitable MPPT method is disclosed in the commonly-owned and co-pendingapplication cited above.

One sample embodiment of the present processes begins by synchronization202 of calculations to the MPPT algorithm. Each time the MPPT algorithmchanges the DC power draw, the synchronization step 202 can be repeated.Following this process, a calculation of changes in DC power and ACvoltages are performed in block 204. Calculation 204 can includedetermining a correlation product (so-called because as power movesdown/up, correspondingly voltage goes down/up), which is equal to ΔDCpower times ΔAC voltage, where A is a change in the variable. Thus, ifpower increases slightly in power distribution grid 110, the AC voltagecan also increase in three-phase AC 108 during islanding detection. Inaddition, if power decreases in power distribution grid 110, the ACvoltage will also decrease in three-phase AC 108.

In block 206 a rolling window filter is established for the lastpredetermined number of correlation products. To allow for a suitablesignal-to-noise ratio, this rolling window filter can maintain e.g. thelast 48 correlation products. Therefore, if the result of the sum of thelast predetermined number of correlations is over such a predeterminedthreshold, it is assumed that islanding has been detected.

At block 210, violation checking of the correlation threshold 208 isperformed. Thus, if a positive violation of the correlation threshold208 has occurred, the inverter 106 is immediately shut down at step 212to prevent any damage, otherwise, the process can cycle and return toblock 202. The shut down 212 can typically be performed in about one totwo seconds.

In a second example, an MPPT algorithm is not used, as in e.g. DC to ACconversion. Modifications in the algorithm as shown in FIG. 2 can beneeded to introduce changes in the power exported; that is, the poweroutput of the converter is caused to fluctuate. Observation of thesefluctuations can be used as described above in connection with an MPPTalgorithm, and the same methods for islanding detection can be used.

According to some but not necessarily all embodiments, there isprovided: Methods and systems for islanding detection in powergenerators, particularly converters. The present innovative methods candisconnect the power converter from the power distribution grid.Islanding detection can be performed by detecting the difference inresistive impedance between the islanded and un-islanded conditions,using an input-output power and input-output voltage correlationtechnique. The present methods can shut down the power converter insteadof allowing it to continue running without connection to the grid, whichwill protect line-worker safety.

According to some but not necessarily all embodiments, there isprovided: A method for islanding detection in a power convertercomprising: making continual changes in DC power draw of a powerconverter which comprises a DC input and an AC output; sensing the ACoutput voltage of the converter responding to each change in the DCpower draw; calculating a correlation product by multiplying the changein DC power draw by the corresponding change in the AC output voltage;summing a predetermined number of correlation products over a rollingwindow to produce a correlation sum; and, for each rolling window,detecting if the predetermined correlation sum has exceeded apredetermined threshold; and shutting down the power converter if thepredetermined correlation sum has exceeded the predetermined threshold.

According to some but not necessarily all embodiments, there isprovided: A power converter comprising: a DC to AC inverter; astored-program controller; wherein the stored-program controllerexecutes instructions to control switches in the inverter; wherein theswitches further control the DC current draw of the inverter and the ACoutput voltage of the inverter; wherein the stored-program controllerfurther executes instructions to: make continual changes in the DC powerdraw of the inverter; sense the AC output voltage of the inverterresponding to each change in the DC power draw of the inverter;calculate a correlation product by multiplying the change in DC powerdraw by the corresponding change in the AC output voltage; sum apredetermined number of correlation products over a rolling window toproduce a correlation sum; and, for each rolling window, detecting ifthe predetermined correlation sum has exceeded a threshold; and, if thepredetermined correlation sum has exceeded the predetermined threshold,then shutting down the power converter.

According to some but not necessarily all embodiments, there isprovided: A power converter comprising: a controller; a first array ofbidirectional switches, connecting lines of a first port to a firstterminal of a paralleledd inductor plus capacitor combination, in eitherof two opposite directions, regardless of the polarity of the externallines at the first port; a second array of bidirectional switches,connecting lines of a second port to a second terminal of a paralleledinductor plus capacitor combination, in either of two oppositedirections, regardless of the polarity of external lines at the secondport; voltage sensors connected to the external lines; at least onecurrent sensor connected to the inductor; wherein the controlleroperates the switches repeatedly to couple power into the inductor fromthe first port, and thereafter to isolate the inductor from all of theexternal lines, and thereafter to couple power out from the inductor tothe second port, and thereafter to isolate the inductor again; andwherein the controller monitors voltage-current relations at one or moreexternal lines, and indicates an islanding condition when the impedanceof one of the external lines has increased detectably, and reduces poweroutput when an islanding has occurred.

According to some but not necessarily all embodiments, there isprovided: A method for islanding detection in a power-packet-switchingconverter, comprising: monitoring the impedance of an output terminalwhich is connected to the power grid; detecting an islanding conditionwhen the impedance seen at the output terminal is lower than that of thegrid; and not powering the output terminal if the islanding condition isdetected.

MODIFICATIONS AND VARIATIONS

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given. It is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

Additional general background, which helps to show variations andimplementations, may be found in the following publications, all ofwhich are hereby incorporated by reference: U.S. Pat. No. 6,603,290; US20120013376; US 20080204044.

None of the description in the present application should be read asimplying that any particular element, step, or function is an essentialelement which must be included in the claim scope: THE SCOPE OF PATENTEDSUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none ofthese claims are intended to invoke paragraph six of 35 USC section 112unless the exact words “means for” are followed by a participle.

Additional general background, which helps to show variations andimplementations, as well as some features which can be implementedsynergistically with the inventions claimed below, may be found in thefollowing US patent applications. All of these applications have atleast some common ownership, copendency, and inventorship with thepresent application, and all of them, as well as any material directlyor indirectly incorporated within them, are hereby incorporated byreference: U.S. Pat. No. 8,406,265, U.S. Pat. No. 8,400,800, U.S. Pat.No. 8,395,910, U.S. Pat. No. 8,391,033, U.S. Pat. No. 8,345,452, U.S.Pat. No. 8,300,426, U.S. Pat. No. 8,295,069, U.S. Pat. No. 7,778,045,U.S. Pat. No. 7,599,196, US 2012-0279567 A1, US 2012-0268975 A1, US2012-0274138 A1, US 2013-0038129 A1, US 2012-0051100 A1; US Provisionals61/765,098, 61/765,099, 61/765,100, 61/765,102, 61/765,104, 61/765,107,61/765,110, 61/765,112, 61/765,114, 61/765,116, 61/765,118, 61/765,119,61/765,122, 61/765,123, 61/765,126, 61/765,129, 61/765,131, 61/765,132,61/765,137, 61/765,139, 61/765,144, 61/765,146 all filed Feb. 15, 2013;61/778,648, 61/778,661, 61/778,680, 61/784,001 all filed Mar. 13, 2013;61/814,993 filed Apr. 23, 2013; 61/817,012, 61/817,019, 61/817,092 filedApr. 29, 2013; 61/838,578 filed Jun. 24, 2013; 61/841,618, 61/841,621,61/841,624 all filed Jul. 1, 2013; 61/914,491 and 61/914,538 filed Dec.11, 2013; 61/924,884 filed Jan. 8, 2014; 61/925,311 filed Jan. 9, 2014;61/928,133 filed Jan. 16, 2014; 61/928,644 filed Jan. 17, 2014;61/929,731 and 61/929,874 filed Jan. 21, 2014; 61/931,785 filed Jan. 27,2014; 61/932,422 filed Jan. 28, 2014; and 61/933,442 filed Jan. 30,2014; and all priority applications of any of the above thereof, eachand every one of which is hereby incorporated by reference.

The claims as filed are intended to be as comprehensive as possible, andNO subject matter is intentionally relinquished, dedicated, orabandoned.

What is claimed is:
 1. A method for islanding detection in a power converter comprising: making continual changes in DC power draw of a power converter which comprises a DC input and an AC output; sensing the AC output voltage of the converter responding to each change in the DC power draw; calculating a correlation product by multiplying the change in DC power draw by the corresponding change in the AC output voltage; summing a predetermined number of correlation products over a rolling window to produce a correlation sum; and, for each rolling window, detecting if the predetermined correlation sum has exceeded a predetermined threshold; and shutting down the power converter if the predetermined correlation sum has exceeded the predetermined threshold.
 2. The method for islanding detection in a power converter of claim 1, wherein the continual changes in the DC power draw of the converter are carried out by a maximum-power point tracking method.
 3. The method for islanding detection in a power converter of claim 1, wherein the number of correlation products for each rolling window is chosen so that the correlation sum exceeds a predetermined signal-to-noise ratio.
 4. The method for islanding detection in a power converter of claim 1, wherein the number of correlation products is about
 48. 5. The method for islanding detection in a power converter of claim 1, wherein the changes in the DC power draw of the converter are caused periodically without regard to maximum-power point tracking.
 6. A power converter comprising: a controller; a first array of bidirectional switches, connecting lines of a first port to a first terminal of a paralleled inductor plus capacitor combination, in either of two opposite directions, regardless of the polarity of the external lines at the first port; a second array of bidirectional switches, connecting lines of a second port to a second terminal of a paralleled inductor plus capacitor combination, in either of two opposite directions, regardless of the polarity of external lines at the second port; voltage sensors connected to the external lines; at least one current sensor connected to the inductor; wherein the controller operates the switches repeatedly to couple power into the inductor from the first port, and thereafter to isolate the inductor from all of the external lines, and thereafter to couple power out from the inductor to the second port, and thereafter to isolate the inductor again; and wherein the controller monitors voltage-current relations at one or more external lines, and indicates an islanding condition when the impedance of one of the external lines has increased detectably, and reduces power output when an islanding has occurred.
 7. A power converter comprising: a DC to AC inverter; a stored-program controller; wherein the stored-program controller executes instructions to control switches in the inverter; wherein the switches further control the DC current draw of the inverter and the AC output voltage of the inverter; wherein the stored-program controller further executes instructions to: make continual changes in the DC power draw of the inverter; sense the AC output voltage of the inverter responding to each change in the DC power draw of the inverter; calculate a correlation product by multiplying the change in DC power draw by the corresponding change in the AC output voltage; sum a predetermined number of correlation products over a rolling window to produce a correlation sum; and, for each rolling window, detecting if the predetermined correlation sum has exceeded a threshold; and, if the predetermined correlation sum has exceeded the predetermined threshold, then shutting down the power converter.
 8. The power converter of claim 7, further comprising: sensors responsive to the DC current draw of the inverter; and, sensors responsive to the AC voltage output of the inverter.
 9. The power converter of claim 7, where the stored-program controller further executes instructions for a maximum-power point tracking method.
 10. The power converter of claim 7, where the stored-program controller is a field-programmable gate array. 